Lots of little demons will prowl the streets tomorrow night -- trick-or-treaters out for a bit of Halloween fun. And a demon star will look down on them. It's called Algol, from the Arabic name "Ras al Ghul" -- head of the demon. So in addition to its proper name, it's also called the demon star.

Algol is in the constellation Perseus, the hero, which climbs the eastern sky during the evening hours. In mythology, Perseus rescued the princess Andromeda from a sea monster. He turned the monster to stone by showing it the head of Medusa, a creature whose head was covered with snakes -- a demon if ever there was one. Algol is Medusa's head.

Algol probably came to represent Medusa because of its behavior: Every 30 hours, its brightness drops by two-thirds. Other stars flicker a bit, too, but no other bright star changes so much and so quickly.

Algol is really a pair of stars locked in a mutual orbit. One of the stars is small and hot, while the other is bloated and cool. Every two-and-a-half days, the smaller star passes in front of the other as seen from Earth, so the system fades.

Even though the dips aren't demonic, Algol would be a scary place to visit. The two stars are so close together that the smaller one is stealing gas from its companion. So a river of fire runs between them -- something you might expect from a stellar demon.

We'll talk about some astronomical ghosts tomorrow.

Script by Damond Benningfield, Copyright 2010

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